Understanding Batteries and Charging Systems

To first understand battery charging a few definitions are in order to
explain the charging phases that are used

Introduction

My interest in this subject came from the desire to build my own emergency power back up system for use with my amateur radio setup. This article is a summary of the information learned from the reference sources below and was written to share with other interested folks in my local amateur radio club. This information has also helped me better understand the performance and limitations of the battery and charging system in my vehicle.

To
understand battery charging systems, a few definitions are in order to explain the
charging phases that are used. Not all battery charging systems use all the
modes explained below.

Modern
lead acid batteries will last longer and charge faster if they are charged in a
particular sequence known as a three step charge. A fourth sequence known as
equalization charge which can triggered by the user exists only in more
sophisticated charging systems.

Bulk Charge

The
first step, known as bulk charge, delivers a constant current rate (at or near
the rated capacity of the charger) until the battery voltage approaches it
gassing voltage typically around 14.4 volts. Depending on the ratio of the
charging current to the ampere hour capacity of the battery, the battery will
be charged anywhere from 75 - 95% of the battery's capacity. (See paragraph
below on charging rate for more detail.)

Absorption Charge

The
second step, known as the absorption charge, holds the voltage constant,
typically at the above mentioned 14.4V, with the current dropping on a slowly
decaying exponential curve.Most of the
relatively inexpensive chargers that have the absorption charge mode hold the
voltage at this state until the charging current has dropped to approximately
10% of the rated capacity of the charger. This charge mode is needed to assure
the last 15 - 20% of the battery capacity is fully charged.

More
sophisticated charging systems use a microprocessor to monitor the history of
the rate of current drop to determine when to exit this phase. In addition they
will have temperature compensating circuit that alters the charging voltage
during this phase.High performance
charging systems will also contain remote sensing of the battery temperatures.
These features are necessary to avoid a thermal runaway condition, which under
the mildest of failure modes cause loss of electrolyte out of the battery and
generally shorting the life of the battery due to internal heating.Rare but worse case failure modes include
battery fires.More discussion of this
topic follows later.

Float Charge

The
third step, known as float charge, holds the voltage approximately at 13.2 -
13.5 volts, enough to maintain the batteries charge without losing electrolyte
through gassing. At this voltage the charge can be left connected to the
battery indefinitely without risk of overcharging.It is interesting to note that most automotive charging systems
are set to have a maximum charging voltage of about 14.2 volts tapering down to
about 13.5 volts. These systems also need to have temperature compensation;
otherwise, either under charging or overcharging will occur depending upon the
temperature of the battery.

Equalization Charge

The
fourth step, found only on the most sophisticated systems is known as the
equalization charge. An equalization charge is needed generally only on flooded
or wet cell batteries (not gel cells) as often as recommended by the battery
manufacturer.The purpose of this
charge is to equalize the capacity of the individual cells of the battery,
reverse electrolyte stratification (the separation of the liquid electrolyte
into layers of different acid concentrations) and break up any residual sulfate
which may remain after normal charging.The charge cycle is in essence a controlled overcharge that tends to
restore the battery back to its original charge capacity. Typically the charge
mode involves taking the charge voltage to at minimum about 15.3V and in some
cases as high as 16.2V. Needless to say this is not something to be done with
out close monitoring or a charging system designed for this purpose, given the
thermal runaway condition that could develop. Also this is not something to be
done with sensitive electronics attached, whether it be your amateur radio
equipment or with the battery installed in a vehicle, given the numerous
computer chips found in today's cars.

Commercially
available, large chargers usually found at full service stations can apply this high
a voltage, not something to be done with in the battery in the car for two
reasons; the risk of damage to the a fore mentioned computers and electronic
devices and, battery "slobbering" (gassing at a high rate that
deposits acid droplets) on top of the battery and adjacent metal, paint and
other components near your battery.

What kind of charger do you have?

There
are 1-Mode, 2-Mode and 3-Mode chargers.

1-Mode

This
charger operates only in float mode. This charger can charge a battery to the
100% level, but it can take days due the lower voltage, and subsequent low
charge current used).

This
type of charger is designed to recharge a battery to 100% more quickly than a
2-mode type. It uses bulk, absorption and float modes.

Charging Rates, Drawing power from the battery
while charging

What if you want to draw power from the battery while it is
being charged?

It
can be done, but you need to know the modes used by your charger and some
problems to be avoided.

If
the charger is a 2-mode, the charger should be large enough to supply the
leakage current of the battery plus any draw from the external circuit
(assuming it is a long term sustained loaded as opposed to an intermittent
load. This is to avoid having the external load from over discharging the
battery.

If
the charger is a 3-mode, you need to read the reference material below to avoid
getting into a situation where the charger locks up in absorption mode and
either overcharges the battery or initiates a thermal run away condition
previously discussed.

Charging Rate

The
fundamental question here is how big a charger do you need for your particular
size (ampere rating) of your battery.

The
maximum battery charging rate is often specified by manufactures for example as
C/5, where C refers to the ampere hour rating of the battery. C/5 for a 100ah
battery would be and optimum charge current (in bulk mode) of 20 amps.The size of an off the shelf charger should
be generally chosen so that the C/5 rate is not exceeded.

This
summary was compiled from a number of sources on the world wide WEB. Thanks
and techncial credits go to these sources for enlightening us all.Further reading is recommended to understand
in greater detail the topics covered.